An indium arsenide photodiode, when irradiated by optical energy, generates a current that is directly proportional to the photon rate of the optical energy incident thereon, i.e., the magnitude of current generated by the photodiode is directly proportional to the number of photons per second incident on the photodiode. Transimpedance amplifiers have typically been used to derive voltages indicative of the current generated by indium arsenide photodetectors. Such amplifiers are employed because they have very low input impedance.
Typical lead wires for indium arsenide photodetectors are fabricated of gold and have diameters of approximately 1 mil, resulting in a resistance of approximately 10 ohms per foot. Lead wires having these characteristics are compatible with indium arsenide bonding pads because of the minimum pressures exerted by the lead wires on the bonding pads and because of the compatibility of gold with other materials of the indium arsenide photodetectors. The small gold wires also minimize heat conduction in cryogenic environments where the photodiode is cooled. For laboratory measurement purposes, the lead wires typically are 1 to 1 1/2 feet long; for operational measurement purposes, the lead wires are a few inches long. Hence, the resistance of the lead wires between the terminals of an indium arsenide photodetector and input terminals of a transimpedance amplifier is significant.
In a typical prior art configuration, the lead wires of an indium arsenide photodiode are connected to inverting and non-inverting input terminals of an operational amplifier having a feedback resistor connected between an output terminal and inverting input terminal of the amplifier. The operational amplifier derives a voltage which is theoretically directly proportional to the current derived by the indium arsenide photodiode and the photon rate of optical energy incident on the photodiode.
In test situations we have encountered using the aforementioned prior art circuit, it has been found that the output voltage of the amplifier is not directly proportional to the photon rate of optical energy incident on the indium arsenide photodetector. We have found that as the temperature of the indium arsenide photodetector varies, the output voltage of the amplifier does not track the photon rate of the optical energy incident on the photodetector. The error in the derived output voltage is particularly significant when it is considered that the dynamic range of an indium arsenide photodiode extends over five decades, from one nanoampere to about 10,000 nanoamperes.
It is, accordingly, an object of the present invention to provide a new and improved circuit for deriving a voltage that accurately indicates the current derived from a variable current generating source.
Another object of the present invention is to provide a new and improved circuit for deriving an output voltage that accurately represents the photon rate of optical energy incident on a photodiode, particularly an indium arsenide photodiode.
A further object of the invention is to provide a new and improved circuit for accurately indicating the current derived from an indium arsenide photodetector that is subject to different temperatures.